1. Field of the Invention
This invention relates generally to an electrophotographic printing system, and more particularly concerns a microcontroller based xerographic charge device power supply.
2. DESCRIPTION OF THE PRIOR ART
The basic xerographic process comprises exposing a charged photoconductive member to a light image of an original document. The irradiated areas of the photoconductive surface are discharged to record thereon an electrostatic latent image corresponding to the original document. A development system next moves a developer mix of carrier granules and toner particles into contact with the photoconductive surface. The toner particles are attracted electrostatically from the carrier granules to the latent image forming a toner powder image. Thereafter, the toner powder image is transferred to a sheet of support material. The sheet of support material then advances to a fuser which permanently affixes the toner powder image thereto.
Before the photoconductive member can be exposed to a light image, the photoconductive member must be charged by a suitable device. This operation is typically performed by a corona charging device. One type of corona generator consists of a current carrying wire enclosed by a shield on three sides and a wire grid over and spaced apart from the open side of the shield. A uniform potential is applied to the wire and the wire grid. Electrostatic fields develop between the charged wire and the shield, between the wire and the grid, and between the charged wire and the (grounded) photoconductive member. Electrons are repelled from the wire and the shield resulting in a charge at the surface of the photoconductive member. The wire grid, located between the wire and the photoconductive members, because of the field between the grid and the wire, helps control the charge strength and uniformity on the photoconductive member caused by the other aforementioned fields.
The control of the charge strength and uniformity on the photoconductive member is very important because consistent high quality reproductions are best produced when a uniform charge is obtained on the photoconductive member. If the photoconductive member is not charged to a sufficient potential, the electrostatic latent image obtained upon exposure will be relatively weak and the resulting deposition of development material will be correspondingly lessened. As a result, the copy, produced therefrom, will be faded. If, however, the photoconductive member is overcharged, the converse will occur and too much developer material will be deposited on tile photoconductive member. As a consequence, the copy produced will have a gray or dark background instead of the white background of the copy paper. Areas intended to be gray are black. Tone reproduction is poor. Additionally, if the photoconductive member is overcharged, the photoconductive member can be permanently damaged.
In a typical xerographic charging system, the amount of voltage obtained at the point of electrostatic voltage (ESV) measurement of the photoconductive member is less than the amount of voltage applied at the point of charge application. In addition, the amount of voltage applied to the corona generator required to obtain a desired constant voltage on the photoconductive member must be increased or decreased according to various factors which affect the photoconductive member. Such factors include the rest time of the photoconductive member between printing, the voltage applied to the corona generator for the previous printing job, the copy length of the previous printing job, machine to machine variance, the age of the photoconductive member and changes in the environment.
Historically, the only factor corrected in applying a voltage on the corona generator to obtain a uniform voltage at the photoconductive member was a rest recovery correction factor. The rest recovery factor attempted to correct for the fact that the photoreceptor responds to charges differently after it is allowed to rest at which time no charge is applied. Preferably, the manner of adjusting the voltage at the corona generator was to adjust the voltage applied to the wire grid.
For example, it would not be uncommon at the end of a 200 copy job for the corona charging device of a copier to generate 1200 volts to obtain 900 volts at the point of measurement on the photoconductive member as measured by an electrostatic voltmeter. After allowing the copier to remain idle for 15 minutes, the corona generator might then need to put out only 1000 volts to obtain 900 volts on the photoconductive member.
Although the classical rest recovery factor has proven beneficial in the control of the charge strength and uniformity on a photoconductive member, there is a need to correct the great many factors which affect the charge strength and uniformity on a photoconductive member.
The problems with typical xerographic charging control systems are not limited to the difficulties associated with rest recovery. In a typical charge control system, the point of charge application, and the point of charge measurement is different. The zone between these two devices loses the immediate benefit of charge control decisions based on measured voltage error since this zone is downstream from the charging device. This zone may be as great as a belt revolution or more due to charge averaging schemes. This problem is especially evident in aged photoreceptors because their cycle-to-cycle charging characteristics are more difficult to predict. The problem results in improper charging, often leading to early photoreceptor replacement. Thus, there is a need to anticipate what the next cycles behavior will be and compensate for it beforehand.
Other difficulties with typical xerographic charging control systems are the calculation and communication requirements placed on a central or main controller. The main controller is often burdened with general process control, diagnostic, and communications requirements that increase the possibility of software crashes and noise induced error signals that undercut the charging function performance as well as the overall machine performance.
The prior art is replete with various charging control techniques. For example, U.S. Pat. No. 4,796,064 discloses a control device for adjusting the surface potential of an image bearing member during the initial cycles of a job run wherein the image bearing member manifests varying characteristics after completion of a job run. The control device includes logic circuitry having means to predict changed characteristics of the image bearing member ,after completion of a first job run at the initiation of a second job run and means to determine a relationship between a charging current of a charging member and a measured surface potential of the image bearing member. More specifically, the control device predicts the charging characteristics of the image bearing members as a function of a rest recovery and a cumulative sum of previous jobs.
U.S. Pat. No. 4,806,980 discloses a feedforward process control for an electrophotographic machine wherein an initial voltage level and an exposure level are process control parameters of the machine. Signals are produced and stored having values characteristics of: (1) a level of at least one of the parameters; and (2) a bias voltage level. A comparison signal is produced by comparing the signal values of charges and the sensed parameters associated with the latent images with the stored signal values for the corresponding latent charge images. Compensation algorithms are used to compensate for noise and disturbances in the initial charge. The effect of the noise and disturbances affects the results.
A difficulty with prior art systems is generally that xerographic power supplies are designed to interface with a central control board through dedicated signal wires. The wiring harness interconnecting each xerographic power supply to the control board must generally support control analog signals (0-10 V), and monitor signals such as analog signals (0-10 V), a digital fault status signal and a digital enable signal. The number of wires is further multiplied by the numbers of xerographic power supplies used in a high volume machine. The resultant wiring harness is a significant contributor to the machine level cost and quality concerns. In addition, the analog signals can be easily contaminated by noise signals propagated throughout the machine environment.
In addition, xerographic power supplies, by nature, represent a hostile environment to digital electronic integrated circuits. The coexistence of 5 V digital controller signals along side high voltage (up to 10 KV) generator signals dictate prudent consideration at the onset of design. Arc discharges, common within the xerographic process, posses a significant risk of catastrophic disruption to the operation of control circuitry. Past experiences have demonstrated the susceptibility of digital electronics to the conducted and radiated energy generated by an arc discharge.
It would be desirable, therefore, to overcome the above identified difficulties in the prior art. It is an object, therefore, of the present invention to provide a xerographic charge device power supply incorporating a microcontroller within the power supply to provide direct local process control and digital communication to link the main controller in the machine. Another object of the present invention is to reduce the wiring requirements to and from the power supply while increasing the communications capability of the power supply. Still another object of the present invention is to provide a charging device power supply that eliminates an external signal conversion printed wiring board. Another object of the present invention is to provide a charging device power supply incorporating internal diagnostic and supervisory functions for communication to the main controller. Other advantages of tile present invention will become apparent as the following description proceeds, and the features characterizing the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.